Cylindrical rolls are utilized in a number of industrial applications, especially those relating to papermaking. Such rolls are typically employed in demanding environments in which they can be exposed to high dynamic loads and temperatures and aggressive or corrosive chemical agents. As an example, in a typical paper mill, rolls are used not only for transporting a fibrous web sheet between processing stations, but also, in the case of press section and calender rolls, for processing the web sheet itself into paper.
Typically rolls used in papermaking are constructed with the location within the papermaking machine in mind, as rolls residing in different positions within the papermaking machines are required to perform different functions. Because papermaking rolls can have many different performance demands, and because replacing an entire metallic roll can be quite expensive, many papermaking rolls include a polymeric cover that surrounds the circumferential surface of a metallic core. By varying the polymer or elastomer employed in the cover, the cover designer can provide the roll with different performance characteristics as the papermaking application demands. Also, repair, regrinding or replacement of a cover over a metallic roll can be considerably less expensive than the replacement of an entire metallic roll.
In many instances, the roll cover will include at least two distinct layers: a base layer that overlies the core and provides a bond thereto; and a topstock layer that overlies and bonds to the base layer and serves the outer surface of the roll (some rolls will also include an intermediate “tie-in” layer sandwiched by the base and top stock layers). The layers for these materials are typically selected to provide the cover with a prescribed set of physical properties for operation. These can include the requisite strength, elastic modulus, and resistance to elevated temperature, water and harsh chemicals to withstand the papermaking environment. In addition, covers are typically designed to have a predetermined surface hardness that is appropriate for the process they are to perform, and they typically require that the paper sheet “release” from the cover without damage to the paper sheet. Also, in order to be economical, the cover should be abrasion- and wear-resistant.
Rubber rolls such as couch rolls, lumpbreaker rolls, forming rolls and press rolls are used in different sections as mentioned above (see, e.g., Pulp and Paper Manufacture (Vol. 7) in Paper Machine Operations, editors Michael J. Kocurek and Benjamin A. Thorpe (1991)) for a discussion of the locations of such rolls in a typical papermaking machine). Rubber rolls typically have excellent chemical, mechanical, physical properties and good abrasion resistance. Also, soft rubber compounds (i.e., between about 30 and 300 on the Pusey and Jones (P&J) scale) ordinarily have excellent dynamic properties under dynamic nip conditions. Polyurethane (PU) is also used to cover rolls for different sections of a papermaking machine. PU covers typically have excellent abrasion resistance, release and toughness compared to rubber, particularly in the hardness range of 4 to 70 P&J. However, PU tends to be expensive, and softer PU (P&J of about 70 to 200) typically has poor chemical resistance compared to rubber.